1. Field of the Invention
The present invention relates to a process for separating molybdenum-99 from neutron-irradiated uranium and also to an apparatus therefor.
2. Description of Prior Art
Molybdenum-99 is produced either by neutron irradiation of molybdenum-98, which constitutes about 25% of the natural molybdenum isotopes, or by fission of uranium-235. The former process is more often used, but in this case, to obtain molybdenum-99 of the high specific activity required for practical use, molybdenum-98 should be irradiated under the condition of thermal neutron flux higher than 10.sup.14 n/cm.sup.2.sec.
In the latter process, by irradiating uranium under the condition of lower neutron flux around 10.sup.13 n/cm.sup.2.sec, molybdenum-99 can be produced at a relatively high fission yield of about 6% and the obtained molybdenum-99 is almost in carrier free state. In this case, however, because of the presence of fission products other than molybdenum-99, the radioactivity of neutron irradiated uranium becomes extremely high, and accordingly, it is technologically very difficult to separate from such highly radioactive uranium rapidly the pharmaceutically acceptable molybdenum-99, the half life of which is as short as 67.0 hours, and recover the same in a high yield.
The methods of separating molybdenum-99 from irradiated uranium are currently known to be commercially applicable; one is "solvent extraction" (see R. E. Lewis. International Journal of Applied Radiation and Isotopes, 22, 603-605, 1971) and the other is the process described in Japanese Patent Public Disclosure No. 16095/73 which is characterized by coprecipitating the molybdenum with .alpha.-benzoinoxim in the form of its complex. But from the viewpoints of maintenance, safety and economy, neither method is fully acceptable. First of all, not a few steps such as precipitation, filtration and extraction should be taken after irradiated uranium is dissolved, and secondly, these steps are not only cumbersome but they result in a great amount of radioactive wastes in liquid form.
As explained in the foregoing paragraphs, the conventional methods for producing molybdenum-99 have many defects to be eliminated, so a process and an apparatus that produces molybdenum-99 in fewer steps more rapidly and safely has been desired for many years by those skilled in the art.
The present inventors, focusing upon the sublimation property of molybdenum trioxide (MoO.sub.3), have carried out serious research on the conditions suitable for the sublimation and separation of molybdenum-99 from irradiated uranium, and as a result, they have developed a process and an apparatus for separating the molybdenum more conveniently, rapidly and safely.